The present invention relates generally to photopolymerized printing plates useful in both letterpress and offset lithography printing processes. More particularly, the present invention concerns water developable photopolymer printing plates which include (a) a support substrate coated with an ink-repulsive layer, and (b) a uniquely formulated adhesive layer which joins separate ink-repulsive and water-soluble photopolymer layers in such a manner as to provide a desired balance between the ink-repulsive and adhesive properties of the resultant printing plate.
In recent years, water-developable photopolymer printing plates have been widely and quite successfully used in various relief printing processes, especially in the newspaper industry, as a consequence of the many practical advantages that water-developable printing plates offer over solvent-developable printing plates. It has long been recognized, for example, that photopolymer printing plates that are developable with organic solvents or aqueous alkaline solutions present a myriad of environmental and plate processing problems that can be readily overcome through the use of water-developable photopolymers.
Yet, despite the widespread acceptance of relief-type, water-developable photopolymer plates, such plates have in the past suffered from the disadvantage of requiring relatively thick photopolymer layers, and, thus, being relatively expensive, when compared, for example, to printing plates that are conventionally used in stereotype systems employed by some large newspapers. A need has arisen, therefore, for less expensive water-developable printing plates, which at least in part can be satisfied through the use of shallow relief photopolymer printing plates.
Such shallow relief photopolymer plates are capable of providing acceptable printing quality with photopolymer layers having greatly reduced thicknesses. This is accomplished through the use of relatively thin photopolymer layers that have raised image or relief areas and recessed background (non-image) areas that include an array of small protuberances. The use of background areas containing the array of protuberances has been found, for example, to prevent bottoming from occurring during the printing process, thereby enabling the use of substantially thinner photopolymer layers which, in turn, results in a substantial reduction in the cost of manufacturing such plates.
While shallow-relief photopolymer plates have overcome many of the problems of utilizing water-developable plates in letterpress printing processes, it has also been found that water-developable plates can be successfully used in offset lithographic printing processes, through the use of unique unilayer film structures having a generally continuous minor phase, e.g., a photosensitive material that changes solubility relative to a selected solvent upon exposure to light, and a generally discontinuous major phase, e.g., a particulate material which is neither photosensitive nor soluble in the solvent. Such unilayer photosensitive film structures, which are described in greater detail in co-pending application Ser. No. 815,899, filed July 15, 1977, can be made selectively permeable to fluids upon exposure to light and can provide lithographic printing plates in which ink receptivity and durability of the imaging areas are relatively independent of the exposure technique and developing composition used in making a finished plate.
It is, of course, desirable in both letterpress and offset lithography printing that printing plates utilized in such processes, at least in some cases, have non-image areas that are ink-repulsive. In the case of letterpress plates, the use of ink-repulsive, non-image background areas permits the use of relatively thick photopolymer layers, without the need for background areas containing an array of small protuberances. In addition, the incorporation of ink-repulsive, non-image background areas in letterpress plates not only allows such plates to be used in color printing, but reduces the possibility of obtaining undesired dark background areas during printing, which can sometimes occur when shallow-relief plates of the type described above are used.
Similarly, in the case of offset printing plates, the use of photopolymer plates having ink-repulsive, non-image areas eliminates the need for water as the vehicle to resist deposits of ink in the non-image areas, which, in turn, eliminates the many problems, such as paper waste, and the development of special inks, papers and rollers, that are commonly associated with offset lithography printing.
Heretofore, one of the major obstacles to the use of water-developable photopolymer printing plates having ink-repulsive, non-image areas has occurred as a direct result of the difficulties experienced in obtaining strong adhesion between the water-developable photopolymer layer and the ink-repulsive coatings contained on such printing plate substrates. When the water-developable photopolymer layer was coated directly onto the ink-repulsive layer, for example, poor adhesion often resulted. And, when intermediate adhesive layers were interposed between the photopolymer and ink-repulsive layers, strong adhesion, but poor ink-repulsive properties resulted.
A need has arisen, therefore, for suitable adhesive compositions, useful in both water-developable letterpress and offset printing plates that can provide a balance between highly desirable ink-repulsive properties and satisfactory adhesion between the separate ink-repulsive and photopolymer layers of such printing plates.